The provision of a signal that is proportional to the absolute temperature is desirable for numerous applications. Such signals are also designated as PTAT, proportional-to-absolute-temperature. For this purpose, a switching circuit that performs a so-called on-chip temperature measurement is suitable.
For the purpose of such an on-chip temperature measurement, frequently a voltage difference between two bipolar diodes, which are charged with different current densities, is evaluated. In the present document, bipolar transistors connected as diodes are understood as bipolar diodes. The difference in the voltages across the two diodes corresponds directly to a temperature-proportional voltage VPTAT according to the rule:
      V    Ptat    =                    V                  d          ⁢                                          ⁢          2                    -              V                  d          ⁢                                          ⁢          1                      =                  V        T            *              l        n            ⁢                        I          2                          I          1                    
Here, Vd1 designates the voltage across the first diode wired in the conducting direction, Vd2 designates the voltage across the second diode wired in the conducting direction, the quotient from I2/I1 designates the ratio of the current densities, with which the first or second diodes are charged, and VT designates the temperature voltage of a diode.
To provide such a circuit, it is typical, for example, to provide two current sources scaled relative to each other, which are connected between a power-supply voltage terminal and a reference voltage terminal, each in a current path with a diode biased in the forward direction. Each input of a differential amplifier is connected to an anode of a diode. At the output, a signal is provided that corresponds to the value of the voltage VPtat multiplied by the amplification factor g of the differential amplifier.
If the temperature-proportional signal is to be provided as a digital signal, then an analog/digital converter can be connected to the differential amplifier.
Such an arrangement, however, suffers from so-called mismatches, both between the diodes and the current sources, which unavoidably appear in the mass production of integrated circuits. In addition, the offset of the amplifier also leads to inaccuracy in the measurement. Methods for compensating such non-ideal properties include, for example, so-called component rotation or chopping.
For example, the document by Anton Bakker: “CMOS Smart Temperature Sensors—An Overview”, IEEE Proceedings, Vol. 2, June 2002, gives an overview of temperature sensors, which can be produced in integrated CMOS processing technology. For example, in FIG. 8 of this document, it is proposed to reach a high accuracy of an integrated PTAT generator with a chopper technique, that is, to periodically chop the signals.
The article by A. Bakker and J. H. Huijsing: “Micropower CMOS Temperature Sensor With Digital Output”, IEEE Journal of Solid-State Circuits, Vol. 3.1, No. 7, July 1996, similarly shows a chopped PTAT circuit in FIG. 3, in which a ΣΔ converter is also provided for signal evaluation.
The document “A Switched-Current, Switched-Capacitor Temperature Sensor in 0.6 μm CMOS” by Mike Tuthill, Journal of Solid-State Circuits, Vol. 3.3, No. 7, Jul. 1998, shows an integrated temperature sensor, which is connected in a so-called Switched Current, Switched Capacitor technique.
The article by A. Bakker and J. H. Huijsing, et al.: “A CMOS Smart Temperature Sensor With A 3σ Inaccuracy of +/−0.5° C. from −50° C. to 120° C., IEEE Journal of Solid-State Circuits”, Vol. 40, No. 2, February 2005, shows a temperature sensor with a ΣΔ modulator and chopping principle.
These circuits have in common the property that the error to be compensated is not completely erased. Either a large expenditure in analog circuitry technology is necessary or additional, undesired properties, such as charge injection, circuitry noise, or the like are introduced.
The document U.S. Pat. No. 6,554,469 shows a temperature sensor with four currents and one transistor. Here it is provided to charge the transistor with each of the four currents and to measure the resulting base-emitter voltage. However, an error relative to the linear amplification remains caused by a mismatch of the current sources.
The document EP 1 132 794 A1 describes a method and an arrangement for obtaining a temperature-independent voltage reference. Here, the amplification factor of an analog/digital converter is calculated from a plurality of digital measurement values.